I have an unusual problem with my car overheating.My car has been overheating for quite some time.

A while back, I noticed that if I drove at a higher speed, the temp would go back down to a lower, more normal temp. But driving at about 25-35 in the city, or at a stop light, the temp would rise again. I later made the connection that it wasn't so much the speed, but the RPM, because one day, going about 25, I switched from overdrive to drive, kicking the RPM's up about 500, cooling the car down.

The only thing I have really done is replace the thermostat, and while it kind of helped for the first week or so, it still having the problem. any ideas as to what could be causing this?

 

I would replace the water pump. There were known to be a failure point. The impellers will wear out and there is nothing to move the coolant at slower speeds. As the rpm's increads, what is left of the impellers will force the coolant to circulate.